The present invention relates to an earth boring apparatus and more particularly to an apparatus and method for boring substantially horizontal holes of various sizes between two spaced apart trenches.
Earth boring apparatus of the present type find application in the laying of pipes, conduits, cables or virtually any other type of underground transmission medium normally laid in a trench dug in the ground. When laying such transmission media over any distance, various surface installations such as roads, driveways, bridges or rail lines to name but a few are usually encountered. To avoid disruption of these installations and the expense of broaching them to lay the line, it has proven desirable and indeed necessary in certain cases to be able to bore a hole of adequate size beneath the surface installation to receive the transmission line. Typically, a trench or operating pit is dug on either side of the surface installation, the pit being of adequate size to receive the necessary boring apparatus at either end of the hole. A pilot hole or coring apparatus is then "shot" from one pit to the other to form and align the hole so formed with the incoming transmission line, which will, for the purposes of illustration, be hereinafter referred to as a pipe. A typical method and apparatus by which such boring has been accomplished to date is illustrated in Canadian Pat. Nos. 760,841, 773,006 and 779,148 to Atkins which issued in 1967. Atkins teaches an apparatus and a four step procedure for performing the bore holes. Firstly, a pilot hole is shot from one trench to the other using a pilot hole cutter. Power is supplied by means of a hydraulic power head situate in one trench. The pilot hole is then expanded by pushing spherical expanders having diameters greater than that of the cutter through the hole to compress the surrounding soil. Once through, the pilot hole cutter and expanders are removed and an earth cutter is affixed to the end of the rod string. The earth cutter is then drawn back, being guided by the pilot hole, to incise a bore hole. The direction of movement of the cutter is then reversed, that is, it is pushed back towards the end of the hole, to extrude the plug. A relatively small downturned flange on the trailing edge of the cutter is intended to score the core to reduce friction between it and the surrounding wall when the hole is being incised and to grip the core when the earth cutter is pushed back to thereby extrude the core.
It will be apparent to those skilled in the art that this manner of forming bore holes suffers from a number of disadvantages. For one, a multiplicity of steps is required, adding complexity, prolonging the operation and increasing the wear and tear on the machinery and hence increasing the risk that breakdowns or problems will be encountered. A prolonged procedure results of course in increased costs.
Another disadvantage is that the pilot hole cutter is easily deflected by stones or other obstructions encountered in the soil, necessitating the use of a relatively unreliable procedure whereby a new hole is shot from the opposite trench using a corrector head. The corrector head must be propelled by its own source of power and is shot in the hope of linking up with the partially formed pilot hole. This is obviously an expensive and time consuming procedure.
A further disadvantage lies in the fact that additional equipment is required, including pilot hole expanders of different sizes and shapes, retaining means to hold the expanders in place, a pilot hole cutter and a winch for drawing and guiding the earth cutter back through the bore to extrude the plug.
Yet another disadvantage is that the small downturned flange on the earth cutter is often inadequate to exert the forces necessary to cause extrusion of the plug. This will occur when the plug is composed of soil which is either somewhat unconsolidated, in which case the soil merely flows through the cutter without being extruded, or is a very viscous, heavy medium such as clay, in which case the small flange will be unable to get a sufficient grip on the plug to overcome its frictional engagement with the bore hole wall. In the result, the cutter will merely rescore the surface of the plug while sliding ineffectually back over it. Further, if only a portion of the core at a time is to be extruded, as is often the case, the small flange is relatively ineffective in "biting" off a piece of the core for extrusion.